The present invention relates to a valve timing control system for an internal combustion engine, which variably controls the opening/closing timing of an engine valve in accordance with the engine operating conditions.
Typically, the valve timing control system comprises a housing rotated by power of a crankshaft and a camshaft-side shaft member coaxially mounted thereto, wherein the mounting angle (rotation phase) between the two can be adjusted through links and a lever. Specifically, the housing is formed with radial grooves in which respective movable guides of the links arranged at distal ends are slidably engaged. A base end of each link is rotatably coupled to the lever protrusively provided to the shaft member. An operating rotator is disposed in front of the housing and the shaft member to be rotatable relative thereto. The backside of the operating rotator is formed with a spiral groove in which the movable guides of the links are engaged. The operating rotator is biased by a power spring in the engine rotating direction, and is subjected to a braking force of an electromagnetic brake as required, wherein the relative rotated position of the operating rotator with respect to the housing can be controlled by controlling energization of the electromagnetic brake.
With the valve timing control system, when the electromagnetic brake is not energized, the operating rotator is biased toward the most forward position in the engine rotating direction by the power spring, having the movable guides operated at a radially inside or outside end of the spiral groove. As a result, the housing and the shaft member are held at the maximum retard phase or the maximum advance phase through the links. Then, when the electromagnetic brake is energized, the operating rotator is operated in accordance with the balance between a biasing force of the power spring and a braking force of the electromagnetic brake, having the movable guides displaced radially while being guided by the spiral groove, thus adjusting the rotation phase between the housing and the shaft member through the links. The spiral of the spiral groove is formed to have the diameter reducing continuously from one end to another end.